What Is Nuclear Magnetic Resonance?
Nuclear magnetic resonance is a physical process in which atomic nuclei with non-zero magnetic moments undergo a Zeeman split in the spin energy level under the action of an external magnetic field, which absorbs radio frequency radiation at a certain frequency. Nuclear magnetic resonance spectroscopy is a branch of spectroscopy, whose resonance frequency is in the radio frequency band, and the corresponding transition is the transition of nuclear spins at the nuclear Zeeman level.
Spin of the nucleus
Nuclear magnetic resonance is mainly caused by
The nuclear magnetic resonance spectrum of hydrogen provides three types of extremely useful information:
Since different types of protons have different chemical shifts, chemical shift values are important for distinguishing various types of protons, and determining the type of protons is important for clarifying
The chemical shift of 13C also uses tetramethyl silicon as an internal standard, and TMS = 0 is specified. Compared with the chemical shift of 1H, there are more factors affecting the chemical shift of 13C, but the electron shielding around the spin core is one of the important factors, so any factor that affects the electron cloud density around the carbon core will affect it. chemical shift. The carbon atom is the skeleton of organic molecules, and the hydrogen atom is at its periphery. Therefore, the interaction between carbon nuclei between molecules has a small effect on c, while the structure of the molecule itself and the interaction between carbon nuclei within the molecule have a relatively small effect on c Big. Carbon hybridization, intra- and inter-molecular hydrogen bonding, various electronic effects, conformation, configuration, and type of solvent during measurement, the concentration of the solution, and the acidity and alkalinity of the system will all affect c. At present, there are some approximate methods for calculating c, which can make qualitative or semi-quantitative estimates of c for some compounds, but more comprehensive theories need to be further explored. The following table summarizes the chemical shifts of C in certain groups based on a large amount of experimental data. The carbon in bold type in the table is the object to be studied.
Chemical shifts of some characteristic carbons Carbon type | chemical shift | Carbon type | chemical shift |
|---|
C H4 | -2.68 | Alpha carbon of tertiary (tertiary) | 70 85 |
Linear alkane | 0 70 | Alpha carbon (secondary) of ether | 60 75 |
Level C | 35 70 | Alpha carbon of ether (first order) | 40 70 |
Tertiary C | 30 60 | Alpha carbon (methyl carbon) of ether | 40 60 |
Level C | 25 45 | R C OOH R C OOR | 160 185 |
Level C | 0 30 | R C OCl R C ONH2 | 160-180 |
C H2 = CH2 | 123.3 | Carbonyl carbon | 165 180 |
Ene carbon | 100 150 | Carbonyl carbon | 150-175 |
C HCH | 71.9 | Carbonyl carbon | 150 175 |
Alkyne carbon | 65 90 | Alpha carbon of amines (tertiary) | 65 75 |
Cyclopropane ring carbon | 2.8 | Alpha carbon of amine (secondary) | 50 70 |
( C H2) n 4 7 | 22 27 | Alpha carbon of amine (first order) | 40 60 |
Carbon on benzene ring | 128.5 | Alpha carbon (methyl carbon) of amines | 20 45 |
Aromatics, replacing aromatic carbons in aromatics | 120 160 | Carbon on cyano | 110 126 |
Carbon on aromatic heterocycle | 115 140 | Carbon on isocyano | 155 165 |
-C HO | 175 205 | R2 C = N-OH | 145 165 |
C = C- C HO | 175 195 | RN C O | 118 132 |
carbonyl carbon of -haloaldehyde | 170 190 | Alpha carbon of sulfide (tertiary) | 55 70 |
R2C = O (including cyclic ketone) carbonyl carbon | 200 220 | Alpha carbon of sulfide (secondary) | 40 55 |
Carbonyl Carbons of Unsaturated Ketones and Aromatic Ketones | 180 210 | Alpha carbon of sulfide (first order) | 25 45 |
Carbonyl Carbon of -Haloketone | 160 200 | Alpha Carbon (Methyl Carbon) of Sulfide | 10 30 |
The measurement of spin coupling is called the coupling constant of the spin. It is represented by the symbol J. The value of J indicates the strength of the coupling. The upper left corner of J is usually labeled with a number, which indicates the separation bond between two coupling nuclei , The lower right of J is marked with other information. By its very nature, the coupling constant is the difference between the two nuclear magnetic resonance energies during proton spin splitting, which can be reflected by the difference in the position of resonance absorption, which is the distance between the splitting peaks on the map.
The size of the coupling constant is related to the relative position between the two interacting nuclei. It will weaken quickly as the number of separated bonds increases. Generally speaking, coupling splitting can occur when two protons are separated by less than or equal to three single bonds. When more than three single bonds are separated, the coupling constant tends to zero. For example, in methyl ethyl ketone, three single bonds are separated between Ha and Hb, so coupling splitting can occur between them, while three or more single bonds are separated between Ha and Hb or Hb and Hc, the coupling between them The effect is very weak, that is, the coupling constant tends to zero. However, two protons with double or triple bonds are inserted in the middle, and remote coupling can occur.
Chemical shifts change with changes in the external magnetic field. The coupling constant is different from chemical shift, it does not change with the change of external magnetic field. Because the spin coupling results from the interaction between the magnetic nuclei, it is transmitted through the bonding electrons, and does not involve an external magnetic field. Therefore, when the peaks formed by chemical shifts are not easily distinguished from the coupling splitting peaks, they can be distinguished by changing the external magnetic field. [2]
In the NMR spectrum, the area under the resonance peak is directly proportional to the number of protons that generate the peak. Therefore, the peak area ratio is the relative ratio of the number of different types of protons. If you know the number of protons in the entire molecule, you can get the ratio of the peak area. Calculate the specific number of magnetically equivalent protons in each group. The nuclear magnetic resonance instrument uses an electronic integrator to measure the area of the peak, and it is represented by a continuous step integration curve from the low field to the hafnium field on the spectrum. The total degree of the integral curve is proportional to the total number of protons in the molecule, and the degree of the step curve of each peak is directly proportional to the area of the peak, that is, proportional to the number of protons that produced the absorption peak. The relative integrated value of each peak area can also be directly displayed on the spectrum as a number. If the area of a peak containing one proton is specified as 1, the number on the spectrum matches the number of protons. [2]
The 1H NMR spectrum provides information such as integration curves, chemical shifts, peak shapes, and coupling constants. The analysis of the map is to reasonably analyze the information and correctly derive the structure of the compound corresponding to the map. The following steps are usually used.
Identified impurity peaks In the 1H-NMR spectrum, impurity peaks that are not related to compounds often appear. Before analyzing the spectrum, they should be marked first. The most common impurity peaks are solvent peaks. Solvent peaks can be generated from undiluted solvents in the sample and non-deuterated solvents mixed in the deuterated solvents used for the measurement. To facilitate their identification, the following table lists the chemical shifts of the most commonly used solvents.
Chemical shifts of common solvents Common solvents | chemical shift | Common solvents | chemical shift |
|---|
Cyclohexane | 1.40 | acetone | 2.05 |
benzene | 7.20 | Acetic acid | 2.05 8.50 (COOH) * |
Chloroform | 7.27 | Tetrahydrofuran | () 3.60 () 1.75 |
Acetonitrile | 1.95 | Dioxane | 3.55 |
1,2-dichloroethane | 3.69 | Dimethyl sulfoxide | 2.50 |
water | 4.7 | N, N-dimethylformamide | 2.77,2.95,7.5 (CHO) * |
Methanol | 3.35 4.8 * | Silicone impurities | 1.27 |
Diethyl ether | 1.16 3.36 | Pyridine | () 8.50 () 6.98 () 7.35 |
* Values vary with measurement conditions. |
The other two peaks to be identified are the rotating side peak and the 13C isotope side peak. In the 1H-NMR measurement, a rotating sample tube generates an uneven magnetic field, which causes symmetrical small peaks on both sides of the main peak. This pair of small peaks is called a rotating edge, and the distance between the rotating edge and the main peak varies with the speed of the sample tube Change. Rotating the side peaks in a properly adjusted instrument can be eliminated. 13C and 1H can even combine to produce split peaks. These split peaks are called 13C isotope side peaks. Since the 13C abundance is only 1.1%, the 13C isotope peaks can only be found when the concentration is large or the map is enlarged.
Calculate the corresponding proton number of each group of peaks according to the integral curve. If the number of protons has been directly marked in the spectrum, this step can be omitted.
(3) Determine their assignment based on the chemical shifts of the peaks.
Determine the correlation between groups based on the shape of the peak and the coupling constant.
The method of heavy water exchange is used to identify the active hydrogen. Because the active hydrogen on one OH, one NH2, and one COOH can be exchanged with D2O. The signal of active hydrogen disappears, so comparing the spectra before and after heavy water exchange can basically determine whether the molecule contains active hydrogen.
Comprehensive analysis, infer molecular structure and check conclusions. [2]
The first-level map is relatively simple and can be analyzed directly according to the above aspects, but the order of dissection can be flexibly grasped according to the actual situation. The spectral lines of advanced maps are generally complicated and difficult to analyze directly. In order to facilitate the dissection, it is best to use a reasonable method to simplify the map before analysis. [2]
Decoupling
The 13C NMR principle is the same as the 1H NMR principle, so the 13C and the directly connected hydrogen nuclei also have a coupling effect. Due to the existence of carbon-hydrogen bonds in most organic molecules, the splitting spectral lines overlap with each other, and the spectrum becomes complicated and difficult to identify. Only by decoupling can the spectrum be made clear and legible. The most commonly used decoupling method is the proton (noise) decoupling method. This method uses a double irradiation method. The power of the irradiation field (H2) includes all resonance frequencies of hydrogen in various chemical environments, so the coupling between 13C and all oxygen nuclei can be eliminated, so that only C, H, O, and N In the 13C-NMR spectrum of ordinary organic compounds, the signals of 13C have become single peaks, that is, all unequal 13C nuclei have their own independent signals. Therefore, the method can identify unequal carbon nuclei in molecules. Below is the 13C spectrum of acetone. (A) is the coupling spectrum, (b 13C spectrum of acetone [2]
) Is the proton decoupling spectrum. In the coupling spectrum, carbonyl carbon ( = 206.7) was double-bonded with six hydrogens and split into seven-fold peaks, and carbon ( = 30.7) was single-bonded with three hydrogens and split into four-fold peaks. In the proton decoupling spectrum, the cleavage peaks of the carbonyl carbon and the carbon both became single peaks. Acetone has two identical alpha carbons and one carbonyl carbon. The peak intensity of the alpha carbon is greater than that of the carbonyl carbon. Proton (noise) decoupling carbon spectrum is commonly referred to as carbon spectrum, also known as broadband decoupling carbon spectrum, which is represented by 13C {H}. There are many other ways to decouple, and interested readers should refer to the related monographs. [2]
[2]
